Well perforating apparatuses typically utilize shaped charges for perforating well casing. The individual shaped charges are mounted onto a shaped charge carrier strip in linear fashion. Each shaped charge includes a case, which contains the explosive charge, and a cap, which encapsulates the explosive charge. A seal means is provided between the case and the cap for keeping the explosive charge dry when the shaped charge is lowered downhole. The seal means also prevents the bleed off of internal pressure.
When shaped charges are lowered downhole, they encounter high temperatures which cause the explosive charge to give off gas and increase the internal pressure. At sufficient depths, the increased internal pressure is unnoticed because it is counteracted by the high external hydrostatic pressure of the drilling fluid. Occasionally the shaped charges must be brought back to the surface in the unfired condition. As the shaped charges are brought back up towards the surface, the external hydrostatic pressure on the shaped charge is decreased. If the internal pressure is allowed to build up to some level greater than the external pressure, the case and the cap will separate, resulting in loose charges in the borehole; a situation which requires expensive operations to retrieve the charges.
In unusual circumstances, high temperatures may also be encountered when shaped charges are shipped by commercial carriers. The Department of Transportation (DOT) has expressed concern about the following possible scenario: a truck carrying shaped charges is involved in an accident and catches fire, which heats the shaped charges. This would result in an increase of internal pressure of the shaped charges. The high internal pressure could force the caps off in a projectile manner and increase the danger of the fire. To prevent such a scenario, the DOT has proposed stricter and more costly regulations in shipping shaped charges.
Thus, what is needed is a shaped charge that will bleed off internal pressure before the case and cap separate to prevent the separation of the case and cap in untimely situations.